In rocket propellants, grenades, and various explosive devices, metal powders are often added to increase the overall heat of combustion and otherwise control the rate of burning of a propellant or explosive. (For example, "Metal Powders for Fuel Propellant, Pyrotechnics, and Explosives" Fauth, pp 597-605, and "Explosivity and Pyrophoricity of Metal Powders" Dahn, pp 194-200, ASM Handbook Vol. 7, 1984.) While zirconium and similar powders have been employed in the past, they are extremely hazardous to use due to the pyrophoricity of zirconium powders and the tremendous heat generated by the burning of such powders. Known techniques for producing such zirconium powders involve reduction processes which provide a fairly wide range of particle sizes, some of which can be extremely fine and almost impossible to handle under conditions other than completely inert ambient atmospheres. Also, the wide range of particle sizes which result from most processing operations can give undesirable burning characteristics which are more difficult to predict and control. The normal methods for the formation of powders involve continued mechanical diminution (i.e. grinding, ball milling, impact crushing, etc.) all produce particles which are extremely nonuniform, irregular and contaminated. These methods often form "dust" (e.g. airborne sub particles). This is the nature of these processes. The powder becomes more dangerous to handle as the particles get smaller, and the degree of surface contamination also increases, resulting in variability in ignition and spontaneous combustion problems. Due to the fact that sub micron powders are subject to agglomeration the actual surface area could be considerably larger than if we assume the particles are separate solid spheres and thus create unpredictable performance.
The most common metal used is Aluminum as an addition to solid rocket propellant and explosive devices. The amount of Aluminum can be up to 20% of the total charge. Other metals are also used and these are Magnesium, Titanium and Zirconium. These metals are generally added in form of very fine particles or powders. In most cases however, the full utilization of the theoretical performance of these metal additions has not been achieved for a variety of reasons. In general, the main factors which govern the performance are the same parameters which control the ignition and subsequent combustion of the metal particles. The rate of combustion can vary from burning (Deflagration) to very rapid detonation of the metal as in explosives. These factors are:
1. Size and shape of the metal in fine dispersion.
2. Surface area/volume ratio.
3. Chemical purity of the bulk metal and its surface.
4. The real and apparent density of the metal.
5. Surface contamination resulting from processing or for safety reasons.
6. Nature of the prepared surface which relates to the method of preparation. . . i.e. made by ball milling, grinding or various chemical or electro-chemical methods.
7. The physical properties such as melting point (in the case of Aluminum, the low melting point results in both particle agglomeration and melting prior to ignition and combustion).
Considerable attention has been spent on optimization of the fuel, oxidizer and binder portions of rocket and explosive devices. In all cases, the metal addition has not received similar attention. As a result, what has been available has been essentially what the powder metallurgy industry can produce and this has resulted in less than desired performance. The following are the most desirable characteristic for metal fuel additions:
1. Metals which are uniform in both size and shape such that reliable reproducible ignition and complete combustion can take place.
2. The metal should be produced in very fine state of dispersion.
3. The metal should be completely dense and not porous or agglomerated powders.
4. The surface of the bulk metal should be of high purity, free of contaminants.
5. Very little size variation to minimize the danger of handling and processing.
6. The metal can be manufactured economically and safely in large quantities.